B cell hybridomas producing antibodies which bind to a human epidermoid carcinoma (A431 cells) have been obtained and their antibodies partially characterized. These monoclonal antibodies elicit a number of binding specificities. Three of the antibodies elicit specificity for what appear to be the EGF receptors on A431 cells; two others elicit specificity for epitopes which may reside on protein(s) complexed with EGF receptors on the A431 cell surface. Immuneprecipitation studies are in progress to describe the precise cell surface macromolecules which interact with these monoclonal antibodies, and studies of their cross specificity, if any, with other A431 cellular proteins and with murine proteins are in progress. The five antibodies cited above do not alter EGF binding to cells directly, but some do alter EGF binding when a secondary antibody specific for the monoclonal antibody light and heavy chains is added. Tests will be undertaken to screen for possible direct or indirect physiological effects of these antibodies on the regulatory processes normally controlled by EGF. The antibodies described above are all selected for binding to the extracellular surface of A431 cells. A selection is currently in progress to screen for monoclonal antibodies which interact with intracellular antigenic sites, specifically those which lead to EGF receptor immuneprecipitation.